1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device that is electrically writable, readable, and erasable, and its manufacturing method. In particular, the present invention relates to a structure of a charge accumulating layer in the nonvolatile semiconductor storage device.
2. Description of the Related Art
The market is expanding for nonvolatile memories as one kind of semiconductor memories, in which data can be electrically rewritten and data can be stored even after the power is turned off. Features of a nonvolatile memory lie in that its structure is similar to that of a MOS transistor and a region capable of accumulating charges for a long period of time is provided over a channel formation region. A floating gate type nonvolatile memory holds charges by injecting the charges in a floating gate through a tunnel insulating film over a channel formation region. In a MONOS (Metal-Oxide-Nitride-Oxide Semiconductor) type nonvolatile memory, a trap or a silicon cluster of a silicon nitride film is used as a charge holdback carrier.
FIG. 16 shows a typical structure of a nonvolatile memory. Over a semiconductor film 800 forming a channel formation region, a nonvolatile memory has a first insulating film 801 which is also called a tunnel insulating film, a charge accumulating layer 802 which is also called a floating gate, a second insulating film 803, a control gate electrode 804, a source 805, and a drain 806.
Such a nonvolatile memory can store one-bit data by one transistor. In a case of writing data, voltage is applied between the source 805 and the drain 806 and between the semiconductor film 800 and the control gate electrode 804 to inject charges from the semiconductor film 800 to the charge accumulating layer 802 through the first insulating film 801; then, the charges are accumulated in the charge accumulating layer 802 that is electrically insulated from its periphery. In a case of reading data, information can be read out by utilizing a characteristic that threshold voltage of a MOS transistor changes depending on whether there are charges in the charge accumulating layer 802. That is to say, the information of “0” and “1” can be stored and read. In a case of erasing data, on the contrary, high voltage is applied to the semiconductor film 800 or the source 805 so as to extract the charges from the charge accumulating layer 802 through the first insulating film 801.
The charges are injected in the charge accumulating layer 802 by increasing voltage applied between the semiconductor film 800 and the control gate electrode 804 and using hot electrons (NOR type) or Fowler-Nordheim type (F-N type) tunnel current (NAND type) flowing through the first insulating film 801 by an intense electric field. In the both types, a high electric field is applied between the semiconductor film 800 and the control gate electrode 804; therefore, the charges are injected in the insulating film that is formed to be thin.
The nonvolatile memory having the charge accumulating layer 802 is required to have a characteristic that can hold charges accumulated in the charge accumulating layer 802 for ten years or more in order to assure the reliability. Therefore, the first insulating film 801 and the second insulating film 803 are required to have a high insulating property so that the charges do not leak from the charge accumulating layer 802. A floating gate type nonvolatile memory has difficulty in thinning the first insulating film 801 so that F-N type tunnel current flows therethrough (7 to 8 nm thick in a case of a SiO2 film) and it is difficult to reduce writing voltage and erasing voltage (10 to 20 V). In addition, a MONOS type nonvolatile memory is required to have a silicon nitride film with a comparatively large volume so that a trap or a silicon cluster in the silicon nitride film holds the charge and threshold voltage of a MOS transistor is changed. Thus, it is considered that element miniaturization and voltage reduction have limitation.
A nonvolatile memory in which the second insulating film 803 in FIG. 16 is formed by a plurality of insulating films and a deep trap level is provided at high concentration in order to reduce writing voltage and improve a charge holding characteristic is known (for example, Reference 1: Japanese Published Patent Application No. H11-40682). Moreover, a MONOS type nonvolatile memory in which a charge holding characteristic is improved by controlling hydrogen concentration of silicon nitride used for the charge accumulating layer 802 is known (for example, Reference 2: Japanese Published Patent Application No. 2004-221448).